This invention relates generally to semiconductor devices, and more particularly to wire bonded integrated circuits.
In fabrication of plastic encapsulated semiconductor devices, the electrically conductive pads of an integrated circuit chip are connected to electrically conductive external leads by means of a very thin wire, typically a gold wire. As shown in FIG. 1a, during molding to encapsulate chip 10 and leads 12, using a dielectric plastic molding compound 14, there is a tendency for wires 11 to distort. This lateral distortion of the wires in the direction of flow of molding compound, represented by arrow 15 is referred to as wire sweep. In FIG. 1b, a gold bond wire 11 is formed in an arc shape between the chip pad 13 and the lead finger 12 in order to prevent wire shorting to either the chip edge, or to the supporting chip pad 14. However, as illustrated in FIG. 1c, a wire 21 often does become deformed, not only by mold compound flow, but also by vibration, mechanical damage or other means, and the tendency for distortion is strongly aggravated by decreasing wire thickness and increasing wire length. As control of the arc shape is lost, spacing between wires is no longer in control.
As the trend to increase the number of input/output connections on circuits has continuously increased, the spacing between pads on the chip has decreased, double tiers of bonding pads have been included, and the length of wires has increased. Leads cannot be fabricated with the same high density as chip pads, and therefore wire lengths have increased in order to allow connection between the leads and closely spaced pads on the chip.
Capacitance loading increases directly with increased wire length, and wire lengths are currently approaching one centimeter. As the wires are brought closer together by design, and/or by wire sweep or other distortion, the separation between wires decreases. All of these factors have a tendency to cause parasitic capacitance coupling between the wires, as well as for short-circuiting of the wires. Increased mutual capacitance between neighboring wires increases electrical noise and affects signal transmission of the circuit. Both self capacitance of long wires and mutual capacitance between wires have become significant obstacles to low cost wire bonded, high speed integrated circuits.
Various attempts have been made to electrically insulate bond wires, by coating with a dielectric material either before or after bonding, and thus to prevent shorting. In several instances wires have been coated with polymers which decompose with heat during the bonding operation. Alternately, thin films of silicones, parylene, other polymers, or even plasma enhanced chemically vapor deposited SiO2 have been applied after wire bonding. However, none have been widely accepted because of deleterious side effects.
Further, almost no attention has been paid to requirements for, or methods to minimize capacitance coupling, and thus improve both reliability and performance of circuits. As the speed of circuits has increased, the parasitic capacitance of wire bonded circuits has become very serious in light of the fact that wire bonded devices, both now and for a some time in the future, will continue to be the economical and preferred method of interconnecting chips to package leads, and therefore a means to minimize the capacitance issues would be very beneficial to the industry.
It is an object of this invention to provide a wire bonded integrated circuit device having low mutual capacitance between the wires, and thus minimize parasitic coupling and cross talk attributable to bond wires.
It is an object of this invention to provide a method for isolating neighboring wire bonds from each other in order to minimize self and mutual capacitance of the wires.
It is an object of the invention to provide a means to insulate bond wires from each other after the wire bonding process has been completed.
It is an object of the invention to provide a means to insulate bond wires from each other after the wire bonding process has been completed, without requiring any cleaning of the leads, internal or external to the molded package.
It is an object of the invention to provide a very low dielectric medium surrounding bond wires which minimizes capacitance coupling between wires, and which eliminates short-circuiting.
It is an object of the invention to provide a dielectric material surrounding bond wires which further effectively has a low modulus of elasticity, and supports enhanced reliability of the device.
It is an object of the invention to decrease mutual capacitance between wires by a factor of about 3 from comparably dimensioned plastic molded wire bonded devices.
The aforementioned objectives are met by first using an electromagnetic model to analyze the capacitance of neighboring wires separated by epoxy molding compound, of wires separated by air only, and of wires separated by a layer of a very low dielectric constant sheath on the wires prior to embedding in a mold compound. Analysis of the data indicates that only a thin layer, approximately 2.5 microns on all sides, of a very low dielectric constant material surrounding the wires will reduce mutual capacitance by a factor of about 3 from that of epoxy molding compound having a dielectric constant of 4. Air as the dielectric for wires at 40 microns separation would provide a 4.5 times decrease from that of molded epoxy. However, because air separation is not a viable solution for plastic molded devices, or even a reliable solution for cavity packages, a very low dielectric constant dielectric constant sheath is provided as a means of minimizing mutual capacitance, and resulting crosstalk.
In order to form a usable, truly low dielectric constant medium surrounding the wire, a foamed polymer having pockets of air or other gas incorporated into the medium is provided. Density of the polymer is decreased, and effectively both the dielectric constant and modulus are decreased by foaming. One method for fabricating such a layer is to react components of a polymer which produce and incorporate gas pockets during curing. Alternate methods for foaming the dielectric medium include adding blowing agents to a polymer prior to curing, thereby capturing air within the medium, and providing the necessary properties. Foamed polymers form low-density embedding materials, reduce the dielectric constant significantly, without creating a rigid coating, such as that found with low dielectric materials, such as polyimides. Further, foamed polymers are processed at temperatures acceptable for wire bonded integrated circuit devices, and they do not require high temperature processing, as do some polymers.